1. Field
One or more embodiments of the present disclosure relate to an autofocus control apparatus.
2. Description of the Related Art
An imaging system employs an autofocus control apparatus to drive a lens in order to allow light to be automatically focused on a stationary image forming plane. Such an autofocus control apparatus may be used to automatically focus light on a surface of an object to be observed and therefore is often found in processing/inspection equipment using a microscope system or other similar optical systems. Accuracy and speed of autofocus control are factors to determine performance of the equipment and thus the quality of products produced by this equipment on a production line.
Examples of methods for performing autofocus control include a positive autofocus control method and a passive autofocus control method.
In the positive autofocus control method, after light beams, reflected at different heights from a focal plane of an objective lens, pass through the objective lens, a focal point of the light beams may vary via a condenser lens located immediately in front of light receiving parts. In this case, the light receiving parts may be located respectively at two positions A and B equidistantly spaced forward and rearward on the basis of a focal point forming location. If the same intensity of light reaches the respective light receiving parts, it may be determined that a sample is at a focal distance.
On the other hand, it may be determined that the sample is located less than the focal distance if the intensity of light measured by the light receiving part at the position A is greater than that at the position B, and that the sample is located beyond the focal distance if the intensity of light at the position A is less than that at the position B. A focal point is recognized based on the above-described principle, and in turn autofocus control is performed based on the recognized focal point.
This method may exhibit relatively fast response time, but relatively low accuracy. In particular, in the case in which a sample is a thin transparent layer, accuracy may be deteriorated due to focal point ambiguity of an objective lens having a particular magnification. This is caused because both beams reflected from upper and lower surfaces of the transparent object are introduced into a light receiving part and a condenser lens focuses the beams at similar positions.
The passive autofocus control method attains images of a sample using an image sensor while moving an objective lens, which is randomly spaced apart from a sample by a predetermined distance on the basis of a reference position. The definitions of several images are then calculated based on a moved position of the objective lens. A position showing the greatest image definition is a focal point of the sample.
Thus, in this passive method, an object to be observed is completely focused when the objective lens is moved from the reference position to the position showing the greatest image definition. This method may accurately find a focal point, but requires a large time to attain and analyze the several images while moving the objective lens by the predetermined distance to find a focal point.